Intensity modulated radiation therapy (IMRT) has evolved toward the use of many small radiation fields, or "beamlets," to increase the resolution of the intensity map. The size of smaller beamlets can be typically about 1-5 cm2. Therefore small ionization chambers (IC) with sensitive volumes < or = 0.1 cm3 are generally used for dose verification of IMRT treatment. The dosimetry of these narrow photon beams pertains to the so-called nonreference conditions for beam calibration. The use of ion chambers for such narrow beams remains questionable due to the lack of electron equilibrium in most of the field. The present contribution aims to estimate, by the Monte Carlo (MC) method, the total correction needed to convert the IBA-Wellhöfer NAC007 micro IC measured charge in such radiation field to the absolute dose to water. Detailed geometrical simulation of the microionization chamber was performed. The ion chamber was always positioned at a 10 cm depth in water, parallel to the beam axis. The delivered doses to air and water cavity were calculated using the CAVRZ EGSnrc user code. The 6 MV phase-spaces for Primus Clinac (Siemens) used as an input to the CAVRZnrc code were derived by BEAM/EGS4 modeling of the treatment head of the machine along with the multileaf collimator [Sánchez-Doblado et al., Phys. Med. Biol. 48, 2081-2099 (2003)] and contrasted with experimental measurements. Dose calculations were carried out for two irradiation geometries, namely, the reference 10x10 cm2 field and an irregular (approximately 2x2 cm2) IMRT beamlet. The dose measured by the ion chamber is estimated by MC simulation as a dose averaged over the air cavity inside the ion-chamber (Dair). The absorbed dose to water is derived as the dose deposited inside the same volume, in the same geometrical position, filled and surrounded by water (Dwater) in the absence of the ionization chamber. Therefore, the Dwater/Dair dose ratio is a MC direct estimation of the total correction factor needed to convert the absorbed dose in air to absorbed dose to water. The dose ratio was calculated for several chamber positions, starting from the penumbra region around the beamlet along the two diagonals crossing the radiation field. For this quantity from 0 up to a 3% difference is observed between the dose ratio values obtained within the small irregular IMRT beamlet in comparison with the dose ratio derived for the reference 10x10 cm2 field. Greater differences from the reference value up to 9% were obtained in the penumbra region of the small IMRT beamlet.

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In order to obtain realistic and reliable Monte Carlo simulations of medical linac photon beams, an accurate determination of the parameters that define the primary electron beam that hits the target is a fundamental step. In this work we propose a new methodology to commission photon beams in Monte Carlo simulations that ensures the reproducibility of a wide range of clinically useful fields. For such purpose accelerated Monte Carlo simulations of 2 x 2, 10 x 10, and 20 x 20 cm2 fields at SSD = 100 cm are carried out for several combinations of the primary electron beam mean energy and radial FWHM. Then, by performing a simultaneous comparison with the correspondent measurements for these same fields, the best combination is selected. This methodology has been employed to determine the characteristics of the primary electron beams that best reproduce a Siemens PRIMUS and a Varian 2100 CD machine in the Monte Carlo simulations. Excellent agreements were obtained between simulations and measurements for a wide range of field sizes. Because precalculated profiles are stored in databases, the whole commissioning process can be fully automated, avoiding manual fine-tunings. These databases can also be used to characterize any accelerators of the same model from different sites.